Equation for the oxidation reaction for KMnO4 solution

[agrocadabra]

Ok so the question goes as such: Write the half equation for the oxidation reaction and then the full net ionic equation for KMnO4 solution mixed with dilute HCL. My main problem is what is getting oxidised?

 

[Borek]

Chlorides.

--
methods

 

[agrocadabra]

chlorides are getting oxidised? how so?

 

[Borek]

It is just a matter of potentials, permanganate in solutions of pH low enough is a very strong oxidiser.

--

 

[agrocadabra]

ok so in this case.. the chlorine will have an oxidation state of minus one to begin with and an oxidation state of -2 to end with?

 

[Borek]

What is oxidation?

--

 

[agrocadabra]

sorry its a loss of electrons.. so the Cl- must lose and electron to become Cl?

 

[agrocadabra]

so the oxidation half equation would be Cl- ---> Cl + e ?

 

[Borek]

Cl₂ to be precise.

See third paragraph here:

http://www.titrations.info/permanganate-titration

--

 

[agrocadabra]

why Cl2 as opposed to Cl? So it oxidises chlorides to chlorine.. why?

 

[agrocadabra]

So the oxidation half equation would be:
2Cl- --> Cl2 + 2e ?

 

[Borek]

Which gases are diatomic?

Edit: yes to your oxidation half reaction.

--

 

[agrocadabra]

But the reduction equation will be: MnO4- + 8H+ + 5e ---> Mn2+ + 4H2O ... when the oxidation and reduction equations are added together the electrons won't cancel..

 

[Borek]

They will, you just have to multiply half reactions by correct coefficients.

Balancing chemical equations with half reactions.

--
methods

 

[agrocadabra]

uhh so multiply all the left side by 2 and the right by 5? then 10 electrons on each and they will cancel?

 

[agrocadabra]

oh no not that easy.. sorry.. i shall read over that link.. thankyou sooo much foir your help so far.. absolute champion!

 

[Borek]

uhh so multiply all the left side by 2 and the right by 5? then 10 electrons on each and they will cancel?

That will probably do - it may happen that coefficients in the reaction after will be not the smallest ones, but at least reaction would be correctly balanced.

--

 

[agrocadabra]

Ok so I get..

2MnO4-(aq) + 16H+(aq) + 10Cl-(aq) ---> 2Mn2+(aq) + 8H2O(l) + 10Cl2(g)

I've got three more of these to do.. ill be up all night.. argh!

 

[Borek]

2MnO4-(aq) + 16H+(aq) + 10Cl-(aq) ---> 2Mn2+(aq) + 8H2O(l) + 10Cl2(g)

Divide everything by 2.

--

 

[agrocadabra]

Ok so the next one is KMnO4 and Na2SO3 in the presence of dilute H2SO4.. however what the heck is the oxidation state of Na2SO3?

 

[agrocadabra]

The oxidation state of the SO3 should be 0.. but what about the Na2? Would the Na2 be assigned a negative oxidation state because of its greater affinity for electrions? so Na is usually + would the oxidation state of Na2 in this case be -2?

 

[Borek]

Don't think in terms of ON, if you are going to balance thorugh half reactions.

Besides, there is no such thing as oxidation state of Na₂SO₃ - oxidation state is assigned to individual atoms, not molecules.

Na₂SO₃ is just a salt - it dissociates giving 2Na⁺ and SO₃^2-. Na⁺ are just spectators.

--
methods

 

[agrocadabra]

so the Na and K ions are spectators in this one?

 

[Borek]

Yes.

--
methods

 

[agrocadabra]

So sulphur is going to be oxidised.. what is the sulfur trioxide's formal charge? is it -2 or 0? if it is -2 then the sulfur has a ON of +4 how the heck do I know what its oxidation number goes to?

 

[agrocadabra]

is the sulfur oxidised to s+6 ions?

 

[Borek]

There is no trioxide here, I have alredy explained what is the ion that will be oxidised.

Yes, +6 will be the final ON of sulfur. No idea why you are using ON - they are not necessary when balancing through half reactions.

--
methods

 

[agrocadabra]

it seems weird that the sulfur would be oxidised... would it be that the SO3 ion is a spectator as well and that the hydrogen ions are oxidised to hydrogen gas?

 

[agrocadabra]

Why is it +6, why not something else, something higher? It just seems so unconcrete.. I'm using ON numbers because I don't know how else to go about it.

 

[agrocadabra]

So it would be S+4 ---> S+6 + 2e?

 

[agrocadabra]

Ok so I get: 2MnO4- + 16H+ + 5S+4 ---> 2Mn2+ + 4H2O + 5S+6

 

[Borek]

No, you can't remove single atom from the complex ion and use it to balance balance reaction. You start with SO₃^2- and you end with SO₄^2-.

Why is it +6, why not something else, something higher?

Because these are properties of sulfuur, properties that you can check looking at the periodic table.

I'm using ON numbers because I don't know how else to go about it.

I gave you a link to a page where half reactions are explained.

--
methods

 

[agrocadabra]

Ok so the third is KMnSO4 solution mixed with dilute H2O2 in the presence of dilute H2SO4:

now the H2O2 is the reductant right.. so we have H+ ---> Hn+ + n electrons.. how do we know what it is oxidised to?

 

[agrocadabra]

You start with SO32- and you end with SO42-. How are there electrons then to cancel out the 5 from the MnSO4?

 

[Borek]

KMnSO₄? No such animal.

Strange as it sounds, hydogen peroxide is oxidised to oxygen.

--